The invention relates to a process for preparing trioxane from a highly concentrated aqueous formaldehyde solution.
Trioxane is generally prepared by reactive distillation of aqueous formaldehyde solution in the presence of acidic catalysts. The trioxane is subsequently extracted from the distillate comprising formaldehyde and water in addition to trioxane using halogenated hydrocarbons such as methylene chloride or 1,2-dichloroethane, or other water-immiscible solvents.
DE-A 1 668 867 describes a process for removing trioxane from mixtures comprising water, formaldehyde and trioxane by extraction with an organic solvent. In this process, an extraction zone consisting of two subzones is charged at one end with a customary organic, virtually water-immiscible extractant for trioxane, and at the other end with water. Between the two subzones, the distillate of the trioxane synthesis to be separated is fed. On the side of the solvent feed, an aqueous formaldehyde solution is then obtained, and, on the side of the water feed, a virtually formaldehyde-free solution of trioxane in the solvent. In one example, the distillate which is obtained in the trioxane synthesis and is composed of 40% by weight of water, 35% by weight of trioxane and 25% by weight of formaldehyde is metered into the middle section of a pulsation column, and methylene chloride is fed at the upper end of the column and water at the lower end of the column. In this case, an about 25% by weight solution of trioxane in methylene chloride is obtained at the lower end of the column and an about 30% by weight aqueous formaldehyde solution at the upper end of the column.
A disadvantage of this procedure is the occurrence of extractant which has to be purified. Some of the extractants used are hazardous substances (T or T+ substances in the context of the German Hazardous Substances Directive), whose handling entails special precautions.
DE-A 197 32 291 describes a process for removing trioxane from an aqueous mixture which consists substantially of trioxane, water and formaldehyde, by removing trioxane from the mixture by pervaporation and separating the trioxane-enriched permeate by rectification into trioxane and an azeotropic mixture of trioxane, water and formaldehyde. In the example, an aqueous mixture consisting of 40% by weight of trioxane, 40% by weight of water and 20% by weight of formaldehyde is separated in a first distillation column under standard pressure into a water/formaldehyde mixture and into an azeotropic trioxane/water/formaldehyde mixture. The azeotropic mixture is passed into a pervaporation unit which comprises a membrane composed of polydimethylsiloxane with a hydrophobic zeolite. The trioxane-enriched mixture is separated in a second distillation column under standard pressure into trioxane and, in turn, into an azeotropic mixture of trioxane, water and formaldehyde. This azeotropic mixture is recycled upstream of the pervaporation stage.
A disadvantage of this procedure is the very high capital costs for the pervaporation unit.
The German patent application DE 103 61 516.4, unpublished at the priority date of the present application, discloses a process for distillatively removing trioxane from trioxane/formaldehyde/water mixtures which does not need extraction or pervaporation steps. However, the process requires a plant with three distillation columns for the removal of pure dioxane and pure water from the product mixture from a trioxane synthesis reactor.